This actually should work. You just need to copy the content of the pem file to the private part of the key when copying it into Termius app in the Keychain section.
We’ll double-check it.
If you have any other problems please contact us here – http://support.termius.com/
ThanksJon Plotter commented
I added the RSA key to the keychain. I selected it after. Yet when I connect, I am asked for the password. Is there anything wrong with what I am doing?
( I saved the key in the password field also - same result)